Silicon substrate having porous oxidized silicon layers and its production method

ABSTRACT

In the silicon substrate having porous oxidized silicon layers of this invention, which consists of a silicon substrate the one surface of which is dotted with porous oxidized silicon layers, the residual internal stress (compression stress) is dispersedly distributed in the porous oxidized silicon layers. Therefore, the entire silicon substrate having porous oxidized silicon layers of this invention is only minimally warped. 
     Adopting a method for producing the silicon substrate of this invention which consists of covering with a mask the surface of a silicon substrate except its dotting areas to be treated, subjecting the silicon substrate to anodic formation in an aqueous hydrofluoric acid solution to form porous silicon layers in the areas to be treated and not covered with the mask and then oxidizing the formed porous silicon layers enables secured production of a silicon substrate dotted with porous oxidized silicon layers.

This application is a division of application Ser. No. 07/556,777, filedJuly 24, 1990.

BACKGROUND OF THE INVENTION

1. Industrial Field of Application

This invention relates to a substrate for the thermal head of a heattransfer printer or a thermal printer, an optical IC, an optical switchor the like, that is to say, a silicon substrate which can be favorablyused as a substrate required to have both heat insulating property andheat radiating property and its production method, and more specificallyto a silicon substrate having porous oxidized silicon layers whichconstitute the heat storage layers of a thermal head, the heatinsulating layers of an optical switch or the like and its productionmethod.

2. Description of the Prior Art

Drawing 18 indicates a conventional thermal head. This thermal head hasan alumina substrate 1 and heat storage layers 2 made of glass glaze andformed on the alumina substrate 1. A heating resistor layer 3 is formedon the heat storage layer 2 and the substrate 1. A conductor layer 4 forfeeding a current to the layer 3 is formed on the heating resistor layer3. One surface of the substrate 1 is dotted with heating parts 6 eachconsisting of a conductor layer 4 and a resistor layer 3. A protectivelayer 5 for protecting the heating parts 6 from oxidation and abrasionis formed over them.

This thermal head is used while being pressed upon a recording mediumsuch as an ink ribbon or a thermal paper (not shown in the drawing). Theink of an ink ribbon can be thermally transferred and a recording mediumcan be colored by heating the heating part 6 of the thermal head byfeeding a current to the heating part 6.

Increasing the amount of heat stored in the heat storage layer 2 made ofglass glaze by increasing its heat capacity by increasing its thicknessis a means for improving the thermal efficiency of such a thermal head.

However, increasing the thickness of the heat storage layer 2 made ofglass glaze results in a prolonged time required for its temperaturedrop after heating and deteriorated thermal response of the thermalhead.

In order to improve the thermal efficiency of the thermal head whileavoiding such a problem, it is recommended to provide heat storagelayers having low thermal conductivity and small heat capacity.

Porous oxidized silicon (POS) is known as a material which can be usedto form such heat storage layers. POS is highly heat resistant and hassufficient mechanical strength.

A thermal head having heat storage layers made of POS has been proposedin Japanese Patent Laid-open No. 257,652/1988. This thermal head has asilicon substrate 11 and a heat storage layer 12 made of POS and formedover the entire one surface of the silicon substrate 11. On the heatstorage layer 12 made of POS, a non-porous oxidized silicon layer 13, aheating resistor layer 3, a conductor layer 4 and protective layers 5are formed in that order.

A known method for producing the silicon substrate 11 of this thermalhead consists of subjecting a silicon substrate 11 to anodic formationin an electrolytic solution (aqueous hydrofluoric acid solution)composed of hydrofluoric acid, alcohol and water to form a layerconsisting of porous silicon (PS) and then thermally oxidizing the PSlayer.

In the above silicon substrate 11, however, since the volume of the PSlayer is increased about 2.2 times during its thermal oxidation intoPOS, internal stress (compression stress) remains in the formed heatstorage layer 12 thereby resulting in great warping of the siliconsubstrate 11.

In practical production of the above substrate 11, forming a POS layer(heat storage layer 12) with 25 μm thickness results in 1 mm warping perinch of the substrate 11. This warping is about 10 times that of acommercially available alumina ceramic substrate. The above siliconsubstrate 11 was of no practical use because of such great warping.

SUMMARY OF THE INVENTION

This invention relates to a substrate for the thermal head of a heattransfer printer or a thermal printer, an optical IC, an optical switchor the like, that is to say, a silicon substrate which can be favorablyused as a substrate required to have both heat insulating property andheat radiating property and its production method, and more specificallyto a silicon substrate having porous oxidized silicon layers whichconstitute the heat storage layers of a thermal head, the heatinsulating layers of an optical switch or the like and its productionmethod.

The object of this invention is to provide a silicon substrate havingporous oxidized silicon (POS) layers and minimally warped and to provideits production method.

BRIEF DESCRIPTION OF THE DRAWINGS

Drawing 1 is a plane view of an example of the silicon substrate havingporous oxidized silicon layers of this invention.

Drawing 2 is an enlarged view of part A in Drawing 1.

Drawings 3-8 are cross-sectional views for describing the processes ofthe production method of Example 2.

Drawing 9 is a cross-sectional view of a thermal head produced inExample 2.

Drawing 10 is a cross-sectional view indicating one process of theproduction method of Example 3.

Drawing 11 is a cross-sectional view of a thermal head produced inExample 3.

Drawings 12-16 are cross-sectional views indicating the processes of theproduction method of Example 5.

Drawing 17 is a cross-sectional view of a thermal head produced inExample 5.

Drawing 18 is a cross-sectional view of a conventional thermal headhaving heat storage layers made of glass glaze.

Drawing 19 is a cross-sectional view of a conventional thermal headhaving a heat storage layer made of porous oxidized silicon.

DETAILED DESCRIPTION OF THE INVENTION

The silicon substrate having porous oxidized silicon layers of thisinvention consists of a silicon substrate the one surface of which isdotted with layers consisting of porous oxidized silicon.

A suitable method for producing this silicon substrate consists ofcovering with a mask the surface of a silicon substrate except itsdotting areas to be treated, subjecting the silicon substrate to anodicformation in an aqueous hydrofluoric acid solution to form poroussilicon layers in the non-covered areas to be treated and then oxidizingthe formed porous silicon layers.

A photoresist or an oxidized film is favorably used as the above mask.

A highly acid resistant one is used as a photoresist. Suitablephotoresists highly acid resistant include negative resists formesa-type etching primarily composed of cyclized-hydrocarbon-systempolymers such as cyclized-butadiene-rubber-system polymers (e.g.,cyclized polybutadienes) or cyclized-isoprene-rubber-system polymers(e.g., cyclized polyisoprenes).

A tantalum oxide (Ta₂ O₅) film or a chromium oxide (Cr₂ O₃) film isfavorably used as the above oxidized film.

Suitable methods for oxidizing porous silicon into porous oxidizedsilicon include the thermal oxidation method of heating the substrate to850°-1,000° C. in an atmosphere containing oxygen and the method ofexposing the substrate to plasma.

Substances which can be used as masks in a method for producing thesilicon substrate of this invention are not restricted to tantalumoxide, chromium oxide and photoresists and also include various passivestate films as well as insulating substances not damaged by hydrofluoricacid such as silicon nitride and sialon. When a silicon nitride film ora sialon film is used as a mask, silicon nitride can be patterned orremoved by dry etching. Molybdenum (Mo), tungsten (W) or a similarmaterial can also be used to form a mask not damaged by hydrofluoricacid. When Mo or W is used for a mask, it is desirable to make the maskthick.

In the silicon substrate of this invention which has dotting POS layers,the residual internal stress (compression stress) is dispersedlydistributed in the POS layers.

EXAMPLE 1

Drawing 1 indicates an example of the silicon substrate having porousoxidized silicon layers of this invention.

The one surface of this silicon substrate 20 is dotted with porousoxidized silicon layers (POS layers) 26. As shown in Drawing 2, theplanar shape of each POS layer 26 is a rectangle having round corners20a. The radius of the round corners 20 is adjusted to 30 μm or above.The size of each POS layer 26 is adjusted to about 1 mm width and 50 mmlength. The distance between POS layers 26 is adjusted to 2 mm or above.

In this silicon substrate 20 which is dotted with POS layers 26, theresidual internal stress (compression stress) is dispersedly distributedin the POS layers 26. Therefore, the entire substrate 20 is minimallywarped.

Furthermore, in this silicon substrate 20 the POS layers 26 of whichhave round corners 20a, cracks are prevented from developing from thecorners 20a of the POS layers 26.

EXAMPLE 2

A method for producing the silicon substrate 20 of the above Example 1using a mask made of tantalum oxide and a thermal head produced fromthis silicon substrate 20 are described according to Drawings 3-9 in thefollowing.

This production method is as follows.

(1) Firstly, a silicon substrate 20 shown in Drawing 3 was prepared. AP-type substrate having a resistivity of 0.01 ω. cm was used as thesilicon substrate 20.

(2) Next, as shown in Drawing 4, a tantalum film 21 was formed on thesilicon substrate 20 by sputtering. It was preferable that the thicknessof the tantalum film 21 be about 0.1-0.5 μm. Parts of the tantalum film21 corresponding to areas 22 in which heat storage layers were to beformed were etched by photolithography to partially expose the surfaceof the silicon substrate 20.

(3) Next, the above tantalum film 21 was thermally oxidized in theatmosphere at a temperature of 500°-1,000° C. to form a tantalum oxidefilm 23 in its surface as shown in Drawing 5. The thickness of thetantalum oxide film 23 depends upon the temperature of the oxidationtreatment. During this thermal oxidation, the silicon surfacescorresponding to the above areas 22 in which heat storage layers were tobe formed were also oxidized into SiO₂ films 24.

(4) Next, the above silicon substrate 20 was subjected to anodicformation using a direct current by placing the silicon substrate 20used as an anode in an aqueous hydrofluoric acid solution of 20 wt %concentration contained in an electrolytic bath in such a manner thatthe substrate 20 faces a platinum plate used as a cathode. The treatmentwas performed at a current density of 50 mA/cm² for 20 minutes.

Since tantalum oxide is in a passive state and is not corroded byhydrofluoric acid, the above tantalum oxide film 23 formed on thesilicon substrate 20 securely serves as a mask. Accordingly, poroussilicon layers (PS layers) 25 having a thickness of 40 μm and a porosityof 80% were formed only in the areas 22 in which heat storage layerswere to be formed and which were not coated with the tantalum oxidelayer 23. The thickness of the PS layers 25 could be freely adjusted bythe time of the anodic formation treatment.

Although dielectric breakdown of the tantalum oxide film 23 during theabove anodic formation is apprehended, since the size of the area 22 inwhich a heat storage layer is to be formed is small, a current densityof about 50 mA/cm² required for anodic formation of the surfaces ofparts of the silicon substrate corresponding to the areas 22 can beobtained only by applying a small voltage of about 0.4 V to thesurfaces. Therefore, there is no possibility that dielectric breakdownof the tantalum oxide film 23 occur during the anodic formation.

(5) Next, after sufficient washing, the tantalum film 21 and thetantalum oxide film 23 over the above silicon substrate 20 were removedby dry etching as shown in Drawing 7.

(6) Next, after sufficient washing, the above PS layers 25 werethermally oxidized at 850°-1,000° C. in a wet atmosphere of oxygen.

Oxidation of the PS layer 25, during which a volume increase occurred asporous silicon (PS) changed into porous oxidized silicon (POS), resultedin the formation a convex POS layer 26 as shown in Drawing 8. The heightof the convexly protruding part of the POS layer 26 was 3-5 μm. Duringthis oxidation treatment, the surface of the silicon substrate 20 wasalso oxidized and as the result a SiO₂ film 24 with 0.2-0.5 μm thicknesswas formed around the POS layers 26.

A non-porous insulating film of non-porous oxidized silicon, sialon orthe like may be formed by sputtering on the thus formed siliconsubstrate 20 as occasion demands.

Drawing 9 indicates a thermal head in which the POS layers 26 of thesilicon substrate 20 produced in the above method are used as heatstorage layers. This thermal head has a heating resistor layer 3 with0.05-0.3 μm thickness formed on the silicon substrate 20 by sputteringof Ta₂ N, Ta-Cr-N, Ta-SiO₂ or the like. A conductor layer 4 with 1-2 μmthickness formed by vacuum evaporation of Al or Ni--Cr/Au is provided onthe heating resistor layer 3. Parts of the conductor layer 4corresponding to the protruding parts of the POS layers 26 have beenremoved by etching to form heating parts 6. The heating resistor layer 3and the conductor layer 4 are connected to each other on both sides ofthe POS layers 26. A protective layer 5 with 5-7 μm thickness formed bysputtering of SiO₂ /Ta₂ O₅, sialon or the like is provided in theoutermost part of this thermal head.

The production method of this example enables the silicon substrate ofExample 1 to be securely produced since a tantalum oxide film used as amask is resistant to an aqueous hydrofluoric acid solution used duringanodic formation.

EXAMPLE 3

The second example of the production method of the silicon substrate isdescribed in the following.

The first half processes of this production method are the same as theprocesses (1)-(4) of the above Example 2.

The production method of this example is different from that of theabove Example 2 in that, immediately after the formation of PS layers 25by anodic formation of a silicon substrate 20 as shown in the aboveDrawing 6, the PS layers 25 were thermally oxidized at 850°-1,000° C.into POS layers 26 without removing the tantalum film 21 and thetantalum oxide film 23 (without performing the above process (5)).

In the thus produced silicon substrate 20, as shown in Drawing 10, theperipheries of the POS layers 26 and areas in which the POS layers 26are not formed are coated with the tantalum film 21 and the tantalumoxide film 23.

Forming a heating resistor layer 3, a conductor layer 4 and a protectivelayer 5 over the silicon substrate 20 by the same methods as in Example2 results in a thermal head shown in Drawing 11.

This production method enables a silicon substrate 20 dotted with POSlayers 26 to be efficiently produced.

EXAMPLE 4

The silicon substrate of this invention was produced by a methoddifferent from the production method of Example 3 only in that chromiuminstead of tantalum was sputtered.

Since a chromium oxide film obtained by oxidizing a chromium film formedby sputtering on a silicon substrate 20 is resistant to an aqueoushydrofluoric acid solution used for anodic formation, this productionmethod enables a silicon substrate 20 dotted with POS layers 26 to beefficiently produced.

EXAMPLE 5

Drawings 12-17 indicate the processes of the fourth production method ofthe silicon substrate of this invention.

(1) In this example, firstly a silicon substrate 20 shown in Drawing 12was prepared. A P-type substrate having a resistivity of 0.01Ω. cm wasused as the substrate 20.

(2) Next, as shown in Drawing 13, a photoresist 32 such as a negativeresist (CBR-M901: manufactured by Japan Synthetic Rubber Inc.) wasapplied to the silicon substrate 20 and then the formed layer waspatterned by photolithography.

(3) After that, subjecting the silicon substrate 20 to anodic formationat a direct current density of 50 mA/cm² for 16 minutes in a 20 wt %aqueous hydrofluoric acid solution contained in an electrolytic bathusing a platinum plate as a cathode and the silicon substrate 20 as ananode resulted in the formation of PS layers 25 having a porosity of 80%and a depth at the opening of the photoresist 32 of 32 μm as shown inDrawing 14. The peripheries of the thus formed PS layers 25 were coatedwith the photoresist 32. The depth of the coated periphery graduallydecreased as it was more apart from the opening of the photoresist 32.

(4) Next, the photoresist 32 was removed to produce the state shown inDrawing 15. The photoresist 32 could easily be removed by a commerciallyavailable exfoliation solution or hot sulfuric acid (sulfuric acid +hydrogen peroxide solution).

(5) Next, after sufficient washing, the PS layers 25 were thermallyoxidized at 850°-1,000° C. in a wet atmosphere of oxygen. Thisoxidation, during which the volumes of the PS layers 25 increased,resulted in the formation of POS layers 26 protruding from the surfaceof the substrate 20. The thickness of the periphery of the POS layer 26gradually decreased. The height of the POS layer 26 was 3-5 μm. As shownin Drawing 16, a SiO₂ layer with 0.2-0.5 μm thickness was formed on thesurface of the silicon substrate 20 surrounding the POS layers 26.

After that, forming a heating resistor layer 3, a conductor layer 4 anda protective layer over the silicon substrate 20 in that order resultsin a thermal head shown in Drawing 17.

According this production method, since the photoresist 32 favorablyweakly adheres to the silicon substrate 20, some of the electrolyticsolution permeates into the area of the substrate 20 coated with thephotoresist 32 and this area undergoes weak anodic formation. As aresult, the thicknesses of the peripheries of the formed PS layers 25and the POS layers 26 obtained by oxidizing the PS layers 25 graduallydecrease outwardly and therefore the concentration of the internalstress of the POS layer 26 in its periphery can be avoided. Accordingly,a silicon substrate 20 produced by this method is of practical usebecause it is minimally warped even in the peripheries of POS layers 26.

What is claimed is:
 1. A method for producing a silicon substrate havingporous oxidized silicon layers comprising the steps of:a. covering thesurface of a silicon substrate with a mask having a pattern to exposeareas of the surface to be treated; b. subjecting the silicon substrateto anodic formation in an aqueous hydrofluoric acid solution to form aporous silicon layer in each of the exposed areas; c. in step a.,utilizing a mask material which, when the silicon substrate is subjectedto the aqueous hydrofluoric acid solution, causes said porous siliconlayer to extend into peripheral covered areas, wherein said poroussilicon layer has a nonuniform thickness that decreases from saidexposed areas to said peripheral areas; and d. oxidizing the formedporous silicon areas.
 2. A method for producing a silicon substratehaving porous oxidized silicon layers as set forth in claim 1, whereinsaid mask consists of tantalum oxide or chromium oxide.
 3. A method forproducing a silicon substrate having porous oxidized silicon layers asset forth in claim 1, wherein said mask is a negative resist primarilycomposed of a cyclized-hydrocarbon-system polymer.